Hi, Ed
Blue-green verdigris is nothing more than flux oxidation of copper. You see it on copper plumbing, also on copper plates purposely exposed to acids in artwork. It's beautiful to the eye of the artist, but horrible as seen by the engineer.
Stranded wires should NEVER be tinned with water soluble or OA fluxes. This is because the flux wicks up under the insulation and is extremely difficult to completely clean out, and this is what typically leads to the oxidation that takes place.
The best practice for tinning stranded wires is to use a process that first dips the wires in IPA, then RMA flux up to one wire diameter below the insulation, then the solder and hold for two or three seconds just under the end of the insulation to allow the solder to wick under the insulation for about one lead diameter, then drop the wires in an ultrasonic bath consisting of 93% hot (140 deg. F) deionized water and 7% saponifier, and then a final DI rinse at 140 deg. F. Do not use IPA in the ultrasonic cleaner (it tends to go BOOM), and do not drop wire assemblies containing circuit boards with wirebonded parts in the untrasonic cleaner, just wires and mechanical terminals.
Not to date myself, but this is the process that was being used when I was just a teenager working for my Dad on the old Apollo spacecraft. The same process was being used through the 60s, thru the 70s when I worked for Honeywell on the Space Shuttle and all sorts of military avionics programs, and is still used today here at GD, at Kongsberg AS, and at many, many other companies. Even if some minor RMA flux residues are left behind, they are so weak and insignificant as to not pose any reliability danger. Once the wires are tinned with this process, they can be soldered to a PWB using water soluble flux and there is little danger of the flux being absorbed into the wire because of the solder plug from the tinning process. I have never heard of a reliability issue with wires tinned using this process, but (like your situation) I have seen many programs grounded from tinning stranded (and even small diameter solid wires) with OA water soluble flux.
Also realize that PTFE cold-flows away completely not only when pinched, but even shrinks just when it is sitting there, and believe me when I tell you that I have lots and lots of experience with that when building to IPC-D-275 and WS6536E where the insulation gap had to be controlled. Changes in temperature can cause the insulation to pull back significantly, revealing the ugly results of tinning with OA fluxes. But in Martha's opinion, that is a good thing. Better to know now than later when the devices begin to fail.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ed Popielarski
Sent: Friday, November 03, 2017 10:39 AM
To: [log in to unmask]
Subject: [TN] Blue-green oxidation on wire
Hi Technetters,
I have 2 questions:
1: How is this oxide formed?
2: is there a "safe" way to clean it after the fact?
On very rare occasion, we have a "verdigris" outbreak on Tin plated solid Cu wire. The insulation tends to not be well adhered (PTFE) when this "breaks out" The stripped wire looks "normal" but when soldered with OA flux (either hand or wave) it starts showing a blue-green oxidation at the insulation/wire interface. Wetting is marginal at best. Scrapping the wire and going to a different manufacturer usually resolves it, but there's still labor involved since it doesn't show up until later in the process & must be reworked.
Thanks in advance!
Regards,
Ed Popielarski
Engineering Manager
[cid:[log in to unmask]]
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Oak Harbor, Wa. 98277
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